A. Field of the Invention
The present invention relates to a method of manufacturing a perpendicular magnetic recording medium, a method of manufacturing a substrate for a perpendicular magnetic recording medium, and a medium and a substrate manufactured by the method.
B. Description of the Related Art
A perpendicular magnetic recording system has been practically applied to achieve high density magnetic recording medium in place of a conventional longitudinal magnetic recording system.
In a conventional longitudinal magnetic recording medium, texture processing is done to facilitate alignment of magnetization in circumferential direction and prevent a magnetic head from adhering to the surface of a magnetic recording medium. The texture processing gives fine streaks in the circumferential direction on a Ni—P plating layer of a nonmagnetic substrate. The texture processing is normally carried out by pressing a polishing tape against the rotating nonmagnetic substrate having a Ni—P plating layer while supplying a slurry of abrasive material.
In a perpendicular magnetic recording system, in which magnetization is aligned vertically, texture traces in the circumferential direction align magnetization in that direction and may inhibit the medium from achieving the low noise performance that is required by magnetic recording. It generally is known in a perpendicular magnetic recording system that higher accuracy in flatness of a substrate surface results in less magnetic noise and better vertical alignment of magnetization. Accordingly, it has been proposed in a series of steps of manufacturing a magnetic recording medium to implement a polishing step on the surface of a nonmagnetic substrate having a Ni—P plating layer composing a magnetic recording medium to achieve a flat surface with high accuracy, while omitting the texturing step in a circumferential direction, as disclosed in Japanese Unexamined Patent Application Publication No. 2005-216465 and 2005-149603, for example.
When the texturing step is omitted in the series of steps of manufacturing a magnetic recording medium, and the polishing step is carried out to obtain a flat surface with high accuracy on the surface of a nonmagnetic substrate having a Ni—P plating layer, the probability of contact increases between the magnetic head and the recording plane of the magnetic recording medium which leads to a degradation in reliability, because of the lack of the texturing step that would have prevented the magnetic head from adhering to the recording plane of the magnetic recording medium. Meanwhile, flying height of a magnetic head has decreased recently in conjunction with enhancement of recording density of a magnetic recording medium. A recording plane of such a magnetic recording medium may suffer from head crash due to abnormal protrusions that may be formed by the polishing step. The head crash damages the recording plane of the magnetic recording medium. On a recording plane of a magnetic recording medium of a perpendicular magnetic recording system that is not processed by texturing, minute protrusions may be formed, which do not lead to head crash, but cause various types of noises during read/write of information by the magnetic head.
Japanese Unexamined Patent Application Publication No. 2005-353177, for example, proposes a method for solving this problem and stabilizing the flying characteristic of a magnetic head. According to this document, after polishing the surface of a soft magnetic underlayer, fine streaks of irregularities are formed in the circumferential direction on the smoothed surface of the soft magnetic underlayer. In this method, fine streaks of irregularities in the radial direction are first formed on the surface of a nonmagnetic substrate composing a magnetic recording medium. Then, a soft magnetic underlayer is formed on this surface by an electroless plating method or the like. Next, the surface of the soft magnetic underlayer is smoothed by polishing. Subsequently on the smoothed surface of the soft magnetic underlayer, the fine streaks of irregularities are formed in the circumferential direction.
In a series of steps of manufacturing a conventional recording medium of a longitudinal magnetic recording system, the purposes of the texture processing include enhancing magnetization alignment in the circumferential direction and preventing a magnetic head from adhering to the surface of the magnetic recording medium. The texture processing for those purposes gives fine streaks in the circumferential direction on a Ni—P plating layer of the nonmagnetic substrate, as disclosed in Japanese Unexamined Patent Application Publication No. 2003-49159. The fine streaks in the circumferential direction are formed by pressing a polishing tape against the rotating nonmagnetic substrate having a Ni—P plating layer while supplying a slurry of abrasive material. If cases where the texture processing generates protrusions or burrs, a means for suppressing these faults has been proposed, which uses a slurry containing abrasive material, organic acid, and water. Japanese Unexamined Patent Application Publication No. 2003-173517 proposes a texture processing that polishes a nonmagnetic substrate, after polishing the substrate surface, using a foamed polyurethane tape and a slurry containing abrasive grains of colloidal silica (having a mean grain diameter in the range of 0.03 to 0.5 μm and a concentration in the range of 3 to 30 wt %) and an additive such as carboxylic acid.
In a series of steps of manufacturing a perpendicular magnetic recording medium, that is, a magnetic recording medium in a perpendicular magnetic recording system, when a polishing step is employed for smoothing the surface of the underlayer of Ni—P alloy without doing texture processing, and fine streaks of irregularities in the circumferential direction are formed on the smoothed surface of the underlayer, an additional step must be done to remove remaining particles (scraped dust), which are abnormal protrusions that can be formed in the polishing step. The polishing step, which leaves a random surface configuration, further involves problems in the flying of the magnetic head
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.